Add DISCARD to the command_no_begin list for AUTOCOMMIT=off.

Backpatch to 8.3.

Reported by Sergey Burladyan.

src/bin/psql/common.c

@@ -1352,6 +1352,9 @@ command_no_begin(const char *query)
 			return true;
 	}
 
+	if (wordlen == 7 && pg_strncasecmp(query, "discard", 7) == 0)
+		return true;
+
 	return false;
 }